The idiom "one bad apple spoils the (whole) bunch", or the alternate form "one bad apple spoils the barrel", means that a bad person, policy, etc, can ruin everything around it. Is there a way to express this in Esperanto?
There is this proverb in Zamenhof’s collection of proverbs:
Unu ovo malbona tutan manĝon difektas
Roughly translated that is:
One bad egg spoils a whole meal
Assuming people know their proverbaro off by heart, it might work to call the person just malbona ovo. Presumably there is some parent saying that English has borrowed as well because of course it’s quite common to call someone a “bad egg” in English too.
The linked page also suggests the following related sayings which seem to have more or less the same meaning:
Plena glaso da vino, sed kun guto da veneno
Unu fava ŝafo tutan ŝafaron infektas
However it’s probably more of a stretch to call the person a guto de veneno or a fava ŝafo.